The present invention relates to furnaces and particularly to apparatus for automatically cleaning spouts that drain molten chemical-containing smelt from the fire box of a chemical recovery furnace.
Wood pulp for paper making is usually manufactured according to the sulfate process wherein wood chips are treated with a cooking liquor including sodium sulfide and sodium hydroxide. The wood chips in the cooking liquor, called "white liquor", are cooked in a digester under predetermined heat and temperature conditions. After cooking, the used liquor, termed "black liquor", containing spent cooking chemicals and soluble residue from the cook, is washed out of the pulp and treated in a recovery unit where the cooking chemicals are refined. Without reclamation and reuse of the cooking chemicals, the cost of the paper-making process would be prohibitive.
In the recovery process, the black liquor is first concentrated by evaporation to a water solution containing about 65 percent solids, which solution is then sprayed into the fire box of a black liquor recovery boiler, a type of chemical reduction furnace. The chemical reduction furnace is a reactor wherein the processes of evaporation, gasification, pyrolysis, oxidation and reduction all occur interdependently during recovery of the cooking chemicals. The organic materials in the black liquor, lignin and other wood extracts, maintain combustion in the fire box, and the heat produced reduces the spent cooking chemicals. A molten smelt flows out of the furnace through a smelt spout to a collection tank. Concurrently, combustion heat is employed to generate steam in a wall of boiler tubes for use as process steam and for generating electricity.
Smelt spouts are designed to drain the smelt from the furnace, and keep the molten smelt within the furnace at a safe level. If a smelt spout is not cleaned periodically, the smelt freezes and forms a crust which will, in time, clog the smelt spout. Clogged smelt spouts can cause the level of smelt inside the furnace to rise, resulting in inefficient and unpredictable furnace operation with an attendant decrease in the amount of chemicals that can be recovered, a decrease in the amount of steam produced per unit of fuel, and increased emission of noxious gases such as carbon monoxide and sulfur dioxide. Moreover, the smelt can build up to a dangerous level and either block furnace air ports, potentially causing the fire to be extinguished, or fill up the furnace windbox, causing serious corrosion problems or even causing smelt to pour out onto the floor adjacent the furnace.
Smelt spouts of chemical recovery furnaces are cleaned manually by a worker inserting a long metal rod into the spout. Vigorous lateral and reciprocating movement of the rod by the worker dislodges char and encrusted material that may be clogging the spout. However, such manual rodding of the smelt spout is inefficient and unsafe and is a tedious, physically demanding job that fatigues operators, which can lead to impaired judgment and reaction time. Shoulder and arm injuries can result from the repeated vigorous motion needed to clean the smelt spout. It is increasingly difficult to find workers to perform such dangerous and demanding tasks.
The temperature of the smelt is 800.degree. to 1000.degree. C., and although the rods utilized to clean the spouts may be in excess of six meters long, there is a danger that a worker who manually rods the smelt spout can be burned if the negative draft of the furnace is lost and flames and/or smelt are emitted from the spout opening. Large pieces of slag inside the furnace can fall and cause the smelt to splash and injure a worker. Smelt spout openings are cooled by water circulating in a jacket surrounding the spout and such water jacket can become ruptured by improper rodding. A broken water jacket can result in an explosion in the furnace.
Other dangers to workers include the potentially hazardous fumes from the collection tank. The above noted attributes of chemical recovery furnaces make it desirable to minimize the amount of time during which workers are near the furnace and exposed to such hazards.
Apparatus for cleaning air ports in a recovery furnace is known, for example as set forth in U.S. Pat. No. 4,423,533 issued Jan. 3, 1984 and assigned to the assignee of the present invention. However, such apparatus is not suitable for cleaning of smelt spouts.
According to U.S. Pat. No. 4,706,324 issued Nov. 17, 1987, also assigned to the assignee of the present invention, a smelt spout cleaner is provided which is mounted upon the smelt spout or immediately thereabove on duct work leading to furnace air ports. Although this equipment has been found very satisfactory, the position thereof in straddling relation to the smelt spout shortens the expected life of the cleaning apparatus not only because of the temperature and splashed smelt, but also as a result of highly corrosive fumes emitted from the "green liquor" tank located below the smelt spout and receiving the smelt spout stream. Even if equipment is plated or powder coated, the corrosive fumes still attack and destroy non-wear parts. Moreover, the apparatus of the +324 patent moves the cleaning head assembly in an arcuate path from a location immediately above the smelt spout down into the smelt spout as suspended via pairs of pivoting arms. Only a relatively short length of the linear smelt spout is completely cleaned on this arcuate path even though some "give" is provided for cleaning head movement by slots in the pivoting arms.
Moreover, even though the cleaner of U.S. Pat. 4,706,324 is manually removable from the smelt spout, as a practical matter such removal must be accomplished at times when the furnace is shut down in view of danger to personnel. Thus, a new smelt spout cleaner cannot be installed, repaired or relocated quickly when the furnace is "on-line". Different furnace operating conditions can lead to different spout cleaning requirements, but the above noted safety concerns do not allow changes to the cleaner with the furnace on-line. Furthermore, the intimate mounting of the smelt spout on the furnace structure requires custom design for each new furnace installation, given the variation in construction between boilers in the design and location of primary air ducts, steam lines, cat walks and other equipment customarily located above the smelt spout.